1. Field of the Invention
The present invention relates to a frame structure for a motorcycle, in which a pair of right and left main pipes and a down pipe extend from a head pipe, and further in which the down pipe and the right and left main pipes are coupled to each other by a stiffener pipe. More particularly, the present invention relates to a frame structure for a motorcycle of the type described which improves rigidity and appearance of the frame, while reducing assembly and material costs.
2. Discussion of the Background Art
As a body frame of a motorcycle, a frame structure constructed by a head pipe, a pair of right and left main pipes, a down pipe, and the like is known (refer to, for example, Japanese Utility Model Laid-Open No. Sho 62-162182, particularly to pp 6-8, FIG. 2 thereof).
FIG. 14 is a side view showing a known frame structure of a conventional prior art motorcycle, reproduced from a figure of the above-referenced Japanese patent publication. Reference numerals in FIG. 14 are re-designated from the original.
In the prior art motorcycle frame structure 200 of FIG. 14, a pair of right and left main pipes 203 and 202 extend rearwardly and downwardly from an upper part 201a of a head pipe 201. A down pipe 204 extends downwardly from a lower part 201b of the head pipe 201, while a head pipe gusset 205 extends from the lower part 201b of the head pipe 201, and a front wall side 206 of the down pipe 204 is supported by the head pipe gusset 205.
The head pipe gusset 205 is a member which is cast integrally with the head pipe 201 and is formed in an almost U shape in cross section by a bottom face 207 and right and left side faces 209 and 208. The head pipe gusset 205 is fitted to the down pipe 204.
By fitting the head pipe gusset 205 to the down pipe 204, the bottom face 207 of the head pipe gusset 205 comes into contact with the front wall 206 of the down pipe 204, the left side face 208 comes into contact with a left side wall 210 of the down pipe 204, and the right side face 209 comes into contact with a right side wall 211 of the down pipe 204.
By welding a periphery 208a of the left side face 208 to the left side wall 210 of the down pipe 204 and welding a periphery 209a of the right side face 209 to the right side wall 211 of the down pipe 204, the down pipe 204 is reinforced by the head pipe gusset 205.
In the case where a load F is applied on the head pipe 201 via the front fork toward the front of the body as shown by an arrow, relatively large tensile stress σ1 is produced in the down pipe front wall 206 and relatively large tensile force σ2 is produced in a rear wall 212 of the down pipe 204. On the other hand, large stress is not produced around the center 253 of the right and left side walls 210 and 211 constructing the down pipe 204.
Consequently, the peripheries 208a, 209a of the head pipe gusset 205 are welded around the center 253 of the down pipe 204, thereby enabling the peripheries 208a and 209a of the head pipe gusset 205 to be welded to portions in which no large stress is produced.
The head pipe gusset 205 is a member which is molded integrally with the head pipe 201 by casting. Therefore, at the time of casting the head pipe gusset 205 in an almost U shape in cross section, a draft angle has to be assured in the right and left side faces 209 and 208.
Therefore, at the time of fitting the head pipe gusset 205 to the down pipe 204, a large gap may be created between the left side face 208 of the head pipe gusset 205 and the left side wall 210 of the down pipe 204 and between the right side face 209 of the head pipe gusset 205 and the right side wall 211 of the down pipe 204.
On the other hand, there is a known conventional frame structure for a motorcycle in which right and left main pipes are welded to a head pipe, and an area around the welded portion is reinforced with a reinforcing member, thereby enhancing rigidity of the portion around the head pipe.
When the reinforcing member is used, however, the number of parts increases and it complicates assembly and parts management.
Consequently, a need exists for an improved frame structure for a motorcycle, achieving simplification of the assembling work process and simplification of parts management by reducing the number of parts required.
In another known conventional frame structure for a motorcycle, a stiffener pipe gusset is welded to a rear wall of a down pipe and a stiffener pipe extends from the stiffener pipe gusset to a main pipe.
However, as described above, when a load is applied on the head pipe, relatively large stress/force is produced in the front and rear walls of the down pipe. Consequently, when the stiffener pipe gusset is welded to the rear wall of the down pipe, relatively large stress may be applied to the welded portion.
Therefore, there is still a need for an improved frame structure for a motorcycle capable of preventing the welded portions between the down pipe and the stiffener pipe gusset from being subjected to large stress. Although the known frame structures for a motorcycle have utility for their intended purposes, a need still exists in the art for an improved frame structure for a motorcycle, wherein rigidity and durability are improved without increasing assembly or material costs. In particular, there is a need for an improved frame structure for a motorcycle that increases rigidity and durability of a motorcycle frame, is easy to assemble and does not increase the number of parts.